The invention relates to a latch needle for a loop-forming machine, in particular for a knitting machine, for the production of flat textiles.
In order to form loops, so-called latch needles are used in machines, in particular in knitting machines, whereby said needles are moved back and forth in rapid succession in order to form loops. In so doing, the latch provided on the knitting machine needle opens and closes a pickup space for a thread in rapid succession and at high speed. This pickup space is limited by the needle base body and by a hook on one end of said needle base body interacting with the opening and closing latch. In so doing, the latch impacts, in rapid succession, alternatively on the hook in order to create the closed position and on the shaft of the latch needle in order to create the open position or rear position. During the loop-forming process, the latch of the latch needle is moved through the loop or the half-loop. The latch speed is highest at the latch tip and decreases as the distance from the fulcrum of the latch—on which it is supported in the knitting machine needle—decreases. The kinetic energy of the latch head at the time of impact must be eliminated on the hook in the closed position and on the needle shaft in open position. To achieve this, a prior-art knitting machine needle 1 in accordance with FIGS. 4, 4a, 5a and FIG. 7 has on the upper side 37 of its needle body 2 a trough-shape recess 27 or a receiving funnel 15 in the form of an impression. Viewed in cross-section, this is essentially followed by a circular arc and is adapted to the back surface 23 of the latch head 8 that is held by the latch shaft 9. On both its sides, the latch shaft 9 terminates on a line 24 in the latch head 8, said line representing the edge of a facet 10, 10′. The faces or facets 10, 10′ extend at an acute angle with respect to each other and adjoin the back surface 23. The latch head 9 is symmetrical with respect to the center axis B and has on its side facing the needle body a width that corresponds to the relatively large width C+ of the recess (FIG. 4a).
This relatively large width C+ of the receiving funnel 15, as well as the large mass of the latch head, can lead to problems because this mass must be moved at high speed back and forth between the closed and rear positions. The kinetic energy must be eliminated on the receiving funnel 15 for the latch head. In the open position, this is achieved by the spring action of the support areas of the shaft jaws, which are also referred to as the jaws of the latch needle. This spring action can be affected by an extension of the latch slit or by an additional, second slit in the latch needle.
The forces created at the time of impact of the latch in rear position may be large enough to break the jaws or the latches.
Another stress, to which a machine knitting needle is subjected during loop formation, is created by the tensile forces of the knit material. The machine knitting needle is guided in a needle channel of a needle bed and its needle underside (back of the needle) is supported on the bottom of the needle channel. This channel may terminate in the throat region, i.e., in the region of the receiving funnel of the latch needle. During the loop-forming operation, the needle moves far beyond the support region of the needle support. In this region, the machine knitting needle is subjected to tensile forces that act in the direction of the needle support. These tensile forces are due to the material takedown. The material take-down already draws knit material away from the needles. The material is held by the needles by half loops located on the needle back. As a result of this, the take-down forces across the half loops act directly on the needle base body which is supported in the needle support. Now, if the half loops are cast off via the closed latches, these tensile forces increase as the half loops move farther in the direction of the needle hook. As a result of this, stress is created that can lead to breakage of the needle shaft. As a rule, the needles break at the receiving funnel or at the mold impression, which represents a weak spot of the latch needle body and is located approximately at the end of the needle support (the so-called cast-off edge), when the knitting machine needle is in moved out position. A mold impression adapted to the prior-art latch head, said impression having a corresponding size, promotes this breaking behavior.
Document DE-OS 28 17 136 discloses a latch needle having a needle shaft with a recess, whereby the shape of the recess is complementary to the back surface of the latch head. The recess is formed by removal of needle shaft material. The support surface of the latch head is limited to the existing width of the lateral walls of the needle shaft, i.e., the needle shaft jaws, whereby the needle shaft is weakened by the removal of the material in order to create the recess. This increases the susceptibility of the latch needle to breakage.
Document DE-OS 22 25 835 discloses a reinforcement of the latch head by means of a lateral rib in order to counteract the wear occurring during the loop-forming process and to counteract the susceptibility to breakage of the latch head. This measure results in an increase of the mass of the latch head and has not gained acceptance in practical applications.
U.S. Pat. No. 2,817,222 has disclosed the reinforcement of the back of the latch head with a rib that sinks into the latch slit in open position of the latch. This measure is to prevent any latch and jaw breakages.
DE Patent 27 14 607 has disclosed a damping of the impact of the needle latch in open position or rear position due to a special design of the longitudinal slit of the needle and to thereby avoid damage to the latch and the needle itself, such damage otherwise occurring in the rear position of the needle latch. To achieve this, a second longitudinal needle slit that has a length extending beyond the end of the opened latch adjoins the first longitudinal needle slit that receives the latch. As a result of this measure, the elasticity of the needle shaft jaws is increased. However, this measure is not sufficient to meet the current requirements expected of high-speed knitting machine needles.